The present invention relates to an electron microscope, and more particularly to an auto-focusing electron microscope suitable for the observation, measurement and/or checking of a circuit pattern such as a semiconductor wafer, a transfer mask or the like.
As the conventional focusing method in an electron microscope is known a method shown in FIG. 1.10 on page 17 of "Scanning Electron Microscopy and X-ray Microanalysis" published by Plenum Press, New York on 1981 or a method disclosed in JP-B-58-48989.
Such a known focusing method in an electron microscope is shown in FIG. 1. In the figure, reference numeral 1 designates an electron gun, numeral 2 a condenser lens, numeral 3 a deflection coil, numeral 4 an objective lens, numeral 5 is a specimen, numeral 6 a specimen holder or table, numeral 7 an electron beam detector, numeral 8 a scanning signal generating circuit, numeral 9 an amplifier, numeral 10 an auto-focusing circuit, and numeral 11 a display. An electron beam generated from the electron gun is focused by virtue of the condenser lens 2 and the objective lens 4 onto the specimen 5 which is mounted on the specimen holder 6. When the specimen 5 has any fluctuation or non-uniformity in height, it is possible to focus the electron beam onto the specimen by changing a magnetization current for the objective lens 4.
When the specimen 5 is irradiated with the electron beam, secondary electrons and/or reflected electrons are generated depending on the composition and/or shape of the specimen and are detected by the electron beam detector 7. An output of the electron beam detector 7 is supplied through the amplifier 9 to a luminance modulation electrode of the display 11. Generally, the scanning signal generating circuit 8 generates sawtooth waves. An output of the scanning signal generating circuit 8 is supplied to the deflection coil 3 so that the electron beam is two-dimensional scanned on the specimen 5. The output of the scanning signal generating circuit 8 is also supplied to a deflection coil of the display 11 to scan a bright spot on the display 11 in synchronism with the scan of the electron beam on the specimen 5 so that an electroscopic detection image of the specimen is obtained on the display 11.
On the other hand, the auto-focusing circuit 10 changes the magnetization current of the objective lens 4 for every scan and receives a detection signal of the electron beam detector 7 to determine an in-focus position from the magnitude of a change in the detection signal or the value of differentiation of the detection signal and to supply to the objective lens 4 a magnetization current with which the electron beam is brought into the in-focus position.
In the above-described prior art, there is a problem that since it is necessary to irradiate the specimen with the electron beam several times until the in-focus position is obtained, the specimen is subjected to its damage and charging by the electron beam, thereby making it impossible to correctly detect a pattern when a semiconductor device or the like is to be observed, measured and/or checked. Also, there arises a problem that any focusing cannot be made for a specimen with no pattern such as a transfer mask for semiconductor device having a protection film formed thereon.